In LTE (Long Term Evolution) systems and LTE-Advanced systems, DRX (Discontinuous Reception) control is introduced for battery saving at user equipment (UE). In LTE standard, the DRX control is defined for user equipments in an idle state and user equipments in a connected state, respectively. Even in the DRX in either of the operating states, the user equipment sets a reception period for receiving radio signals transmitted from a base station (evolved Node B: eNB) at a certain constant cycle, and the base station transmits the radio signals to the user equipment in the reception period.
In the LTE systems and the LTE-Advanced systems, the user equipment in the idle state receives a paging message from the base station under the DRX control as follows. In the LTE standard, a paging occasion (PO), which is a subframe for transmitting the paging message, and a paging frame (PF), which is a radio frame possibly including the paging occasion, are determined at both the user equipment and the base station in accordance with the following formula. Namely, the system frame number (SFN) of the PF is determined as follows,SFN mod T=(T div N)*(UE_ID mod N).Here, T is a DRX cycle for the user equipment to receive the paging message and is represented in the number of radio frames. N is the minimum value of T and nB (nB is a value selected from 4T, 2T, T, T/2, T/4, T/8, T/16 and T/32). Also, UE_ID is determined as follows,UE_ID=IMSI mod 1024,where IMSI is the IMSI (International Mobile Subscriber Identity) for the user equipment.
The subframe number of the PO in the PF determined in the above manner is determined from an index i_s and a parameter Ns calculated in accordance with the following formula by using a correspondence table defined in the LTE standard,i_s=floor(UE_ID/N)mod Ns, where Ns is the maximum value of 1 and nB/T.
In this manner, the PF and the PO can be uniquely determined from an identifier (IMSI) of the user equipment recognized by both the user equipment and the base station, and the user equipment in the idle state waits to receive the paging message from the base station in the PO configured in the PF. When the base station transmits the paging message in the PO in the determined periodic PF, the user equipment can receive the transmitted paging message.
On the other hand, the user equipment in the connected state receives a PDCCH (Physical Downlink Control Channel) from the base station under the DRX control as follows. In the DRX control in the connected state, the user equipment is managed with two states, that is, an active period where a communication circuit is activated to receive the PDCCH from the base station and an inactive period where the reception circuit is not activated. In the active state, the user equipment monitors the PDCCH from the base station and transmits feedback information and/or an SRS (Sounding Reference Signal). For example, the feedback information may include a CQI (Channel Quality Indicator), a PMI (Precoding Matrix Indicator), a RI (Rank Indicator), a PTI (Precoding Type Indicator) or the like. On the other hand, in the inactive state, the user equipment does not monitor the PDCCH from the base station and does not also transmit the feedback information and the SRS.
In the LTE standard, the active state is defined as cases where any of an On duration timer, a drx-Inactivity timer, a drx-Retransmission timer and a mac-contention Resolution timer is activated, a scheduling request is transmitted, an uplink grant for uplink HARQ retransmission is assigned, or a random access (RA) response is received but no PDCCH indicative of new transmission is received. On the other hand, the inactive state in the DRX control is defined as cases other than the above-stated cases.
For example, as illustrated in FIG. 1, when the drx-Inactivity timer has expired, the user equipment transitions from the active state to the inactive state and then activates the DRX. As illustrated, during activation of the DRX, the user equipment transitions to the active state at a constant DRX cycle and attempts to receive the PDCCH from the base station in a period (On duration) of the active state.
Also, as illustrated in FIG. 2, upon receiving a DRX MAC (Medium Access Control) CE (Control Element) including state control information from the base station to indicate that the user equipment should active the DRX, the drx-Inactivity timer is stopped in accordance with the DRX MAC CE. In this case, as illustrated, the user equipment transitions from the active state to the inactive state and then activates the DRX. Similar to FIG. 1, during activation of the DRX, the user equipment transitions to the active state at a constant DRX cycle and attempts to receive the PDCCH from the base station in the period (On duration) of the active state.
Meanwhile, in the presently developed 3GPP Rel-13, it is discussed that the DRX to which a longer DRX cycle than the above-stated current DRX is applied may be introduced to suppress the transition from the idle state to the active state, which may entail increasing signaling amounts and latency, for infrequently communicating MTC (Machine Type Communication) terminals. Specifically, the current DRX cycle can be set as up to 2.56 seconds, but it is discussed that it can be set as longer than 5 minutes, that is, an extended DRX (eDRX) is discussed. Also, it is desirable that the eDRX can be applied to both the DRXs for the above-stated conventional idle state and connection state.